It has been a well known art for people to utilize a laser beam, such as semiconductor laser, in reading data from an optical recording media, such as a CD or DVD. The laser beam is focused through an objective lens in the pickup head to aim at the data tracks formed on the optical disc. Then, a photo detector transforms the laser beam reflected from the optical disc into signals corresponding to the data stored on the disc. During the reading process, a tracking signal (TS) and a focusing signal (FS) have to be obtained from the returning beam, so as to control a tracking actuator and a focusing actuator to precisely move the laser beam aiming on the desired track position.
To achieve the servo control of tracking and focusing, the objective lens has to be controllably moved in its axial direction, i.e., the focusing direction, and in the radial direction of the disc, i.e., the tracking direction. Therefore, a lens holder is needed for carrying the objective lens and assuring stable movements of the lens. The related art lens holders are as follows.
a) Wire-supported type lens holder:
As disclosed in U.S. Pat. Nos. 5,381,273; 5,555,623 or 5,587,845, a related art lens holder includes four metal wires, such as stainless steel wires, to support the objective lens. The wires are in parallel to each other, each fixed with one end to a base, and the other ends to a lens holder, so that the lens and the holder can be controlled by a tracking actuator and a focusing actuator to move in two directions for tracking and focusing.
b) Leaf-spring type lens holder:
U.S. Pat. No. 5,892,628 discloses an objective lens holder using four spring plates to support the lens. Also, in the U.S. Pat. No. 5,453,881, only a pair of spring plates are used to support the objective lens.
c) Axial sliding and rotating type lens holder:
As illustrated in U.S. Pat. No. 5,587,845, the lens with its carrier is mounted on a supporting shaft which can slide along the axial direction of the shaft, and swing pivotally on the shaft, by the controls of a focusing actuator and a tracking actuator to achieve the focusing and tracking operations.
The conventional wire-supported type lens holder uses four parallel metal wires to support the objective lens. When the holder is moving for tracking or focusing, the wires are apt to twist and cause inaccuracy to the tracking and focusing operations. In the aforementioned U.S. Pat. Nos. 5,381,273; 5,555,623 and 5,587,845, a sheet metal is cut to form two strips which are further resin molded into a base piece and a bobbin half. Two such base pieces and bobbin halves are then connected into a base unit and a holder part for supporting the objective lens. In such procedures, the metal strips are easier to be made, but the connection process is still complicated.
In the leaf-spring type lens holder of U.S. Pat. No. 5,892,628, four spring plates are used to support the objective lens, but the fabrication process is more difficult.
As for the axial sliding and rotating type lens holder, the tolerance of the lens carrier to the supporting shaft should be precisely controlled, or unfitting will occur and the lens will not be correctly positioned.